That long, sexy, flowing mane on a girl's (or guy's) head. If your hair is neck length, it cannot truly be calassified as long hair. Long hair is usually shoulder blade length or longer.
Long Hair, Extremely popular back in the 80's and become somewhat of a trend for guys, It has slowly died away and less guys now have long hair due to music these days. Long hair is slowly Becoming more popular on guys.
Girl: Hey it's Matt Tuck from Bullet For My Valentine
Guy: What?
Girl: Oh It's just a guy with long hair like him how cool
Guy: Thanks ;)